1. Field of the Invention
The present invention pertains to prestressed concrete structural members and in particular pertains to concrete which is prestressed using post-tensioning techniques.
2. Description of the Prior Art
Prestressed concrete plays a significant role in many of the building structures in use today. Prominent applications of prestressed concrete include: bridges, building columns, and liquid storage tanks. Common to each of these applications, is the goal of eliminating tension forces in concrete load-bearing members, since concrete is notably weak in tension, but is strong in compression. In each of these applications, a prestressing force, applied prior to the concrete being loaded through use, is generated by stretching steel reinforcing members or tendons positioned internal to the concrete member. The stretched reinforcing members exert a compressive force on the concrete, which is arranged (in any one of several different ways) to prevent their relaxing.
The most common tensioning members in use today are made of steel, in the form of bars or wires, called "tendons." The tendons may be stretched through the use of hydraulic jacks or the like, and the prestressing results when the tendons are prevented by the concrete from relaxing, i.e., returning to their initial length. The tendons are usually made of high-strength steel which can satisfactorily maintain high working stresses, typically ranging between 150,000 and 180,000 pounds per square inch. These high tension levels are required to overcome losses or partial relaxation in the tension due to shrinkage and plastic flow of either the tendon or the concrete over time. It is frequently inconvenient to apply stresses of these magnitudes, especially since, prestressing must often be performed in the field. An alternative technique, not requiring the stretching of tendon members, could provide significant economic and safety-related advantages.
Prestressing is commonly accomplished in one of two ways: pretensioning or post-tensioning, and may be applied either to pre-cast members manufactured off site, or may be done in the field, at the point of use of the concrete member. In pretensioning, stretched tendons are mechanically bonded to the concrete while the concrete is being cured. However, in the post-tensioning method, reinforcing members are prevented from being bonded to the concrete, thereby allowing the members to be stretched after the concrete is cured. An example of post-tensioning will follow shortly.
In post-tensioning beams, axially-extending tendons are typically encased in sheaths to prevent bonding of the tendons to the concrete. When the concrete has been cured to a predetermined minimum strength, hydraulic jacks or the like tension the tendons by working against the ends of the beam, thereby putting the beam in compression. The compression is thereafter maintained by anchoring the tendons to the concrete, allowing removal of the hydraulic jacks. Thereafter, grout may be forced into the sheaths to establish a bond with the prestressed concrete beam. Although the grout does not add to the prestressing forces, it does advantageously impart to the prestressed beam a greater reserve strength and better crack control under overload conditions. If cracks should appear under small overloads, they generally will close when the load is removed, thereby adding to the longevity and integrity of the prestressed concrete beam.
In addition to bridges and building structures, prestressed concrete is particularly advantageous when used for liquid-containing storage tanks. Concrete tanks are superior to tanks made of steel or similar materials in that corrosion and the like problems are avoided. The objective of prestressing concrete tanks is to maintain the tank walls in compression even when they are filled with a liquid. In the post-tensioning method of prestressing concrete tanks, a track is placed on top of the concrete tank after a sufficient curing time has elapsed. A wire-winding machine, suspended from a track at the top of the tank, is employed to wrap one or more wire tendons around the cylindrical outer surface of the tank at a typical tension of the order of 150,000 to 180,000 psi. Wire-wrapped tanks are usually coated with a corrosion preventing layer, 1-inch thick or so, of pneumatically applied mortar or cement. More complete descriptions of such post-tensioning methods are given in commonly-assigned U.S. Pat. Nos. 3,687,380 and 4,005,828.
Liquid storage tanks and other concrete structures frequently employ stud-like sealing members which consist of steel bars, cast in the ends of the concrete member. The free ends of the sealing members project outwardly beyond the ends of the concrete structure to provide a means of securing the sealing structure, for example, to the end of the concrete member, by welding or the like joinder. While generally successful for providing a point of attachment, this technique fails to utilize the structural strength of the sealing membranes, typically welded steel, which, as pointed out, are embedded within the concrete tank or shell during casting of the concrete. The sealing membranes are not load-bearing and do not play a role in prestressing the concrete. These same considerations are equally applicable to hollow tubes and to cylindrical column members used as structural elements in buildings.
Prestressing techniques have also been employed in commonly assigned application Ser. No. 06/818,203, filed Jan. 13, 1986. This patent application describes an assembly for the end-wise joinder of multiple sections of a rail gun barrel to form a continuous tube. The joint resists bursting forces as a projectile is advanced through the barrel. This technique is illustrated by the butt-connection of two tubular sections. Each section is flared and has an internal cavity for receiving a pressure medium which compresses the sections together in an axial direction and applies a radially-inward force to resist bursting pressures.
The end sections of the tubes are outwardly flared and contain internally located pressure cavities within the flared region, inclined in the direction of the flare. A collar-like coupling surrounds the end section of the tubes after they are placed together. A hole is then drilled through the coupling into the flared region of each end section so as to communicate with the pressure cavity. A pressure medium, such as a liquid resin, is inserted into the cavity under pressure, and is subsequently allowed to cure so as to be transformed into its solid phase. The pressurized cavity, located in the flared end, is expandable so as to apply axial compressive stresses to the joint as well as radially inwardly-directed forces to resist bursting pressures within the tube. While generally satisfactory for joining two sections of a tube together to form a butt connection, no provision is made for the prestressed construction of a tube-like structural member, such as a hollow prestressed concrete cylinder.